Yeah its totally realistic if you have a great niche (str8 college boy is a great niche), a good converting design and are able to muster up traffic. 500 members total is about 16 billing events a day, given a good quality members area you should be running 2:1 on rebills:signups, so lets say 10 rebills and 5 signups a day.

You could accomplish this as quickly as 3 months, or it could take years too....depending on your traffic, of course.